1. Field of the Invention
This invention relates to an insulator string assembly for overhead transmission lines which assembly includes bullet-proof rings for protecting the individual insulators of the insulator string assembly against bullets.
2. Related Art Statement
To protect insulators of overhead power transmission lines, bullet-proof boards have been used. Typical materials of a conventional bullet-proof board for this purpose are metal plates, bullet-proof glass, polycarbonate sheets, fiber-reinforced plastic (FRP) sheets, special fiber cloths, and the like. Japanese Patent Laid-open Specification No. 66,771/77 discloses some of such materials.
A drawback of such conventional bullet-proof boards is that they are not suitable for protection against high-energy bullets such as 222 Remington bullets. If only steel or other metallic plates are used, the bullet-proof boards for the high-power bullets become very heavy, while if only other materials such as FRP sheet are used, the bullet-proof boards for high-energy bullets become too thick and bulky, for instance three to four times as thick as that in the case of steel plates. Such heavy or bulky bullet-proof boards are not suitable for application to the protection of the insulator strings in overhead transmission lines.